Question

What is the correct output of preprocessing the following 3 lines under the C99 rules?

#define y(x) x
#define x(a) y(a
x(1) x(2)))

BTW cpp under linux produces an error message, but I can’t see why the answer isn’t simply

1 2

Assuming cpp is correct and I’m wrong, I’d be very grateful for an explanation.

Answer 1

When a macro is found, the preprocessor gathers up the arguments to the macro and then scans each macro argument in isolation for other macros to expand within the argument BEFORE the first macro is expanded:

6.10.3.1 Argument substitution

After the arguments for the invocation of a function-like macro have been identified,
argument substitution takes place. A parameter in the replacement list, unless preceded
by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is
replaced by the corresponding argument after all macros contained therein have been
expanded. Before being substituted, each argument’s preprocessing tokens are
completely macro replaced as if they formed the rest of the preprocessing file; no other
preprocessing tokens are available.

So in this specific example, it sees x(1) and expands that, giving

y(1 x(2)))

It then identifies the macro call y(1 x(2)), with the argument 1 x(2) and prescans that for macros to expand. Within that it finds x(2) which expands to y(2 and then triggers the error due to there not being a ) for the y macro. Note at this point its still looking to expand the argument of the first y macro, so its looking at it in isolation WITHOUT considering the rest of the input file, unlike the expansion that takes place for 6.10.3.4

Now there’s some question as to whether this should actually be an error, or if the preprocessor should treat this y(2 sequence as not being a macro invocation at all, as there is no ‘)’. If it does the latter then it will expand that y call to 1 y(2 which will then be combined with the rest of the input ()) and ultimately expand to 1 2